1. Field of the Invention
The present invention relates generally to conversion of organic materials into protein. Specifically, the present invention relates to a process for converting biomass wastes, for example, into high-grade bacterial protein suitable for use as animal feed or human food supplements.
2. Description of the Prior Art
Harmless microorganisms have long been used by man for the production of beer, wine, cheeses, breads, sausages, yogurts, soy sauce, and other foodstuffs. During both World Wars, Germany pioneered work on growing microbes on relatively inexpensive substrates, such as molasses, not to alter the taste of foods, but rather to produce edible microbial cell mass for use as a meat substitute or meat extender. Protein produced from single-celled microorganisms for use as animal feed or human food has since become known as single-cell protein (SCP).
Free-world production of SCP today is about 2.times.10.sup.6 tons per year which is used for food or food supplements. SCP is in economic competition with relatively expensive soybean meal, fish meal, egg, or skim milk proteins for common uses. Pruteen, a commercial production of Imperial Chemical Industries in Great Britain, is a SCP made by non-N.sub.2 -fixing bacteria growing on methanol derived from fossil fuels, as is the case with most other SCP products. The current market price for Pruteen is about 50 cents per pound with about three-fourths of the operating expenses attributable to the costs for methanol, ammonia, and oxygen.
Use of inexpensive biomass waste materials for SCP production would greatly benefit the economics of the process. However, no microbe known will directly convert more than a small fraction of lignocellulosic waste substrates into SCP over a reasonably short period of time. In general, aerobic bacteria convert about 25% of easily digestable substrates into new cell mass, and anaerobic bacteria convert only about 7%. Furthermore, much of the lignocellulosic materials are not easily digested and may take months to be catabolized, if at all.
Each of the five billion people on earth ideally needs about 2600 calories of food per day to maintain health, which is equivalent to about 20 quads of foodstuff energy per year worldwide (about one quad of food energy is ingested per year by the total U.S. population). Approximately 20%, or 12.times.10.sup.6 metric tons, of the U.S. citizen's caloric intake is in the form of protein obtained from animal or vegetable sources. The national requirement for protein is actually considerably larger, since it takes about 10 pounds of feed to make a pound of beefsteak or 3 pounds of feed to make a pound of chicken. It has been estimated that even if all of the available protein worldwide in 1980 had been equitably distributed, there still would have been a shortfall of 10.sup.7 metric tons (I. Goldberg, 1985, Single Cell Protein, Springer-Verlag, New York). The protein shortage will increase 2.5-fold by the year 2000.
Direct and indirect fuel consumption represent a major cost in the production of food. About one-third of the fossil fuel used in agricultural production and its transportation is used by the Haber process for the synthesis of ammonia-based fertilizer. A few leguminous crops (e.g., peas, peanuts, and soy beans) employ a symbiosis with N.sub.2 -fixing bacteria in their roots to partially fulfill their requirement for ammonia. Even so, lack of sufficient ammonia is rate-limiting for growth, and application of additional nitrogenous fertilizer will enhance productivity. Also, growing plant crops to raise food requires long growing seasons.
A method to rapidly produce an inexpensive, high-quality protein product without ammonia fertilization would have immense value and potential primarily for use in producing animal feed, but also, under certain conditions, in producing human food. The capacity for high-rate production would ameliorate the effects of drought, famine, or national disasters. It should also decrease the demands for land and energy currently allocated for growing animal-feed crops. Existing schemes for single-cell protein synthesis involve use of expensive or potentially toxic petroleum-based organic substrates and nitrogenous fertilizers.
There has not heretofore been provided a technique or process for simple and effective conversion or organic materials into protein suitable for use as animal feed or human food supplements.